Ability to pay theories of tax fairness have been ascendant since the late nineteenth century due to a confluence of at least six ideological, political, and institutional factors. Each of these six historical causes corresponds to an independent justification of ability to pay: equality as non-discrimination, equality as redistribution, equal sacrifice of welfare, limited government, political neutrality for social stability, and administrative efficiency. These six widely varying justifications make ability to pay theories highly flexible and allow them to find support from greatly differing moral and political perspectives. Yet this façade of consensus conceals underlying disagreement. The six distinct justifications of ability to pay have significantly different implications for fundamental issues in tax policy. Applying ability to pay in practice requires both judgments about which justifications of ability to pay are most compelling and an account of the role that ability to pay should play in normative reasoning about taxation.